Magnetic resonance imaging device and method

ABSTRACT

When data of plural original images having different echo times are acquired to produce water/fat separated images by performing an processing operations in an MRI apparatus, a partial region of the original image data is specified and the specified region is subjected to the water/fat separation processing. Since noise components included in the specified region are fewer than those in the original image data, errors occurring due to noise components during unwrapping and other such processing operations can be reduced and image quality degradation be suppressed. Therefore, water/fat separated images having an excellent image quality can be produced. When echoes having different echo times are generated, rewind pulses are applied ahead of the readout gradient magnetic fields to equalize the polarities of the readout gradient magnetic fields. Consequently, influence of application of gradient magnetic fields on echo signals can be suppressed and the accuracy of water/fat separation can be improved.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a magnetic resonance imaging(MRI) apparatus and a method of imaging a subject to be examined byutilizing nuclear magnetic resonance (NMR). In particular, it relates toan MRI apparatus and method of producing plural kinds of images bycollecting plural NMR signals with different echo times.

[0002] Prior art

[0003] An MRI apparatus produces images by exciting nuclear spins ofatoms constituting a subject to be examined, mainly protons, with an RFmagnetic field pulse, acquiring signals generated by NMR as echosignals, and performing various processing operations on the echosignals. Images varying in tissue contrast can be obtained by changingparameters such as TE, i.e., echo time from excitation of spins togeneration of echo signals, repetition time TR and so forth, or byperforming image processing operations. In clinical applications, thereis a need for images in which MR signals from fat are suppressed, andvarious techniques for obtaining such images have been proposed and putto practical use.

[0004] Typical methods for obtaining a fat-suppressed image include (1)a selective excitation method using an RF frequency, (2) an inversionrecovery method, and (3) a method using image processing operations.

[0005] Method (1) requires very high uniformity of the static magneticfield generated by a magnet in the space surrounding the subject to beexamined.

[0006] In method (2), while uniformity of the static magnetic field isnot important, signals from tissue with T1 value about the same as thatof fat may be suppressed together with the fat signals and the SNRbecomes low as a whole.

[0007] A typical example of method of (3) is the Dixon method, which isdescribed, for example, in “Simple Proton Spectroscopic Imaging” W.Thomas Dixon, RADIOLOGY, Vol.153, 189-194 (1984).

[0008] The Dixon method is a water/fat separation method that utilizesdifference of chemical shift between the water proton and fat proton.Since the water proton and fat proton precess at different resonancefrequencies f₀w and f₀f, the magnetization vector of the water protonand that of the fat proton come to be oriented in different directionsas time passes. When the difference in resonance frequencies between thewater proton and fat proton is Δf and 2τ=1/Δf, the water proton and fatproton oriented in the same direction at the time of excitation becomeoriented in the opposite direction (180°), in the same direction (360°),. . . alternately at each τ.

[0009] The difference between the frequency of precession movement offat proton and that of water protons is known to be 3.5 ppm and when theresonance frequencies of the water proton and fat proton are representedby f₀w and f₀f respectively, the difference Δf can be expressed by thefollowing formula.

Δf(=f ₀ w·f ₀ f)˜γB ₀×3.5×10⁻⁶

[0010] In the formula, γ is gyromagnetic ratio and B₀ is static magneticfield intensity.

[0011] The Dixon method utilizes the above-mentioned fact that waterproton and fat proton come to have the same phase and opposite phasealternately at every τ; that is, the phase of water MR signals and thatof fat MR signals become the same and opposite alternately.

[0012] The principle of the two-point Dixon method (abbreviated 2PDmethod hereinafter) is shown in FIG. 1. In the 2PD method, measurement(scan) using a gradient echo (GrE) sequence is carried out twice whilechanging TE.

[0013] In FIG. 1, 101 indicates RF excitation pulse, and 102 and 103indicate readout gradient magnetic fields. Slice gradient magneticfields and phase encoding gradient magnetic fields are not shown in thefigure. In the first scan, the echo time TE1 is set to be integral times2τ and a readout gradient magnetic field pulse 102 is applied. In thesecond scan, the echo time TE2 is set to be τ longer than TE1 of thefirst scan and a readout gradient magnetic field pulse 103 is applied.

[0014] The behavior of water proton spin (water signal) and fat protonspin (fat signal) are shown in the lower portion of FIG. 1, where thewater signal is indicated by a black arrow and the fat signal by a whitearrow. At the time of the first scan, the water signal 104 and the fatsignal 105 are in the same phase. On the other hand, at the time of thesecond scan, the water signal 104 and the fat signal 105 are in oppositephases. When the intensities of water signals and fat signals in the(x,y) coordinate of images are represented by W(x,y) and F(x,y),respectively, signal S1(x,y) of the first scan and signal S2(x,y) of thesecond scan can be expressed by the following formulae (1) and (2),respectively.

S1(x,y)=W(x,y)+F(x,y)  (1)

S2(x,y)=W(x,y)−F(x,y)  (2)

[0015] Addition of these formulae (1) and (2), S1(x,y)+S2(x,y)=2W(x,y),gives a water image and subtraction of these formulae,S1(x,y)−S2(x,y)=2F(x,y), gives a fat image.

[0016] Although the GrE sequence is illustrated in FIG. 1, a spin echo(SE) sequence may be employed.

[0017]FIG. 2 shows the case where the SE sequence is employed. When theSE sequence is used, an RF excitation pulse 201 and RF inversion pulse202 are applied at the same timing in two scans. In the first scan, areadout gradient magnetic field pulse 203 is applied and a signal isacquired at TE1. In the second scan, a readout gradient magnetic field204 is applied and a signal is acquired at TE2, which is τ after TE1 ofthe first scan. Then, a water image and a fat image can be produced inthe same manner as explained above.

[0018] According to the 2PD method, if the static magnetic field is nothomogeneous, accurate results cannot be obtained. In MRI apparatuses,the static magnetic field generated by a magnet in the space surroundingthe subject to be examined should ideally be homogeneous but it is notalways homogeneous because of distortion of the magnet. In addition,non-uniformity of the static magnetic field may be produced by thesubject present in the static magnetic field space due to differentmagnetization in the subject. Such non-uniformity of the static magneticfield in a field of view (FOV) causes change of the MR signal frequencyand image degradation, such as position shift, flow artifact or thelike. Further, image phase is changed by non-uniformity of the staticmagnetic field and a correct result cannot be obtained by complexprocessing operations between images. When the static magnetic field isnot homogeneous, the aforementioned formulae (1) and (2) are modified asexpressed by the following formulae (3) and (4).

S1(x,y)=(W(x,y)+F(x,y))exp(iα(x,y))  (3)

S2(x,y)=(W(x,y)−F(x,y))exp(iα(x,y)+α′(x,y))  (4)

[0019] In the formula (4), “α(x,y)” consists of a phase rotationcomponent caused by non-uniformity of the static magnetic fieldgenerated during the time 2τ×n (=TE) and a phase rotation componentcaused by non-uniformity of the RF excitation pulse, and has the samevalue for echoes while depending on position. α′(x,y) is a phaserotation component caused by non-uniformity of the static magnetic fieldgenerated during the time τ.

[0020] Thus, when the static magnetic field is not homogeneous, thenon-uniformity of the static magnetic field generates phase differencebetween water signals of the first scan and water signals of the secondscan and water signals and fat signals cannot be separated by a simpleaddition/subtraction.

[0021] In order to overcome the above problem, an auto-shimmingtechnique is employed to directly correct non-uniformity of the staticmagnetic field in the FOV using additional coils (shim coils) orpost-processing of images is performed to correct for non-uniformity ofthe static magnetic field. The latter method, which is a modified Dixonmethod including phase correction of signals using the static magneticfield distribution, is called a three Point Dixon (3PD) method.Principle of the 3PD method will be explained with reference to FIG. 3.

[0022] In the 3PD method, scan is performed three times while changingTE. The first and second scans are the same as those of the 2PD method.Here, in the first scan, TE1 for an RF exciting pulse 301 is set to beinteger times 2τ and a readout gradient magnetic field 302 is applied.In the second scan, TE2 is set to be τ longer than in the first scan anda readout gradient magnetic field pulse 303 is applied. In the thirdscan, TE3 is set to be τ longer than in the second scan (2τ longer thanin the first scan) and a readout gradient magnetic field 304 is applied.

[0023] Signals measured in the first scan and in the second scan can beexpressed by the above-mentioned formulae (3) and (4), and signalsS3(x,y) measured in the third scan can be expressed by the followingformula (5).

S3(x,y)=(W(x,y)+F(x,y))exp(iα(x,y)+2α′(x,y))  (5)

[0024] In the first scan, the phase of a water signal 305 and that of afat signal 306 become same, which is represented by 307, and has a valueα. In the second scan, the phase of a water signal 308 and that of a fatsignal 309 are opposite and the phase of the water signal comes to havea value α+α′. In the third scan, a water signal 311 and a fat signalcome to have the same phase, which has a value α+2α′. Since the watersignal and the fat signal are in the same phase, the phase rotationamount due to non-uniformity of the static magnetic field can be foundby calculating the phase of S3(x,y)/S1(x,y) using the following formula(6).

arg(S3(x,y)/S1(x,y))=2α′(x,y)  (6)

[0025] In the formula, arg( ) means finding of the phase in theparentheses.

[0026] Phase rotation amount α′(x,y) due to non-uniformity of the staticmagnetic field can be obtained by finding values of the above formulafor every (x,y). Then, the following formulae are calculated using thethus obtained α′(x,y).

S1′(x,y)=S1(x,y)exp(−i2α′(x,y))  (7)

S2′(x,y)=S2(x,y)exp(−i(2n+1)α′(x,y))  (8)

[0027] Since phase rotation amount due to non-uniformity of the staticmagnetic field is corrected for the thus obtained signals S1′, S2′,addition of the above formulae (7) and (8) gives a water image W(x,y)and subtraction of the above formulae (7) and (8) gives a fat imageF(x,y).

S1′(x,y)+S2′(x,y)=2W(x,y)

S1′(x,y)−S2′(x,y)=2F(x,y)

[0028] Not only the GrE sequence but also the SE sequence can be used inthe 3PD method. The same steps as in the SE sequence shown in FIG. 2 areperformed till the end of the second scan and, in the third scan, asignal is acquired by applying a readout gradient magnetic field pulse τlater than in the second scan (2τ later than in the first scan).

[0029] The 3PD method explained above takes three times the ordinaryscan time because the scan must be performed three times. In order toreduce the scan time, there has been proposed a method using a sequencewhere three echoes required for the Dixon method are measured at asingle scan (called a single scan sequence hereinafter). In the singlescan sequence, as shown in FIG. 4, after a first echo signal 402 isacquired at the same time as in the first scan shown in FIG. 3 after anRF pulse 401 is applied, the readout gradient magnetic field is reversedtwice to generate echoes 403, 404 at τ and 2τ after the first echosignal. That is, three echo signals having different echo times areacquired by applying a single RF exciting pulse 401.

[0030] While the GrE type single scan sequence is shown in FIG. 4, theSE type single scan sequence as shown in FIG. 5, in which the first echosignal is measured as a spin echo, can be employed. In this case, an RFinversion pulse 502 is applied at TE/2 after application of an RFexciting pulse 501 and a first echo signal 503 is acquired at TE whileapplying a readout gradient magnetic field Gr. Immediately thereafter,the readout gradient magnetic field is reversed twice to generate echoes504, 505 at τ and at 2τ after the first echo signal. Thus, three echosignals having different echo times are acquired by applying a single RFexciting pulse as in the GrE type.

[0031] As a method other than this type of 3PD, there is known a methodin which a water/fat separated image is produced by finding phaserotation amount due to non-uniformity of the static magnetic field fromtwo signals acquired at TE and TE+τ in the 2PD method. This 2PD methodaccompanied by the correction of the static magnetic field is describedin “Two-Point Dixon Technique for Water-Fat Signal Decomposition with B0Inhomogeneity Correction”; Bernard D. Cooms et al.; Magnetic Resonancein Medicine, Vol.38, 884-889(1997).

[0032] The aforementioned 2PD method (including the method with thestatic magnetic field correction) and the 3PD method have the followingproblems.

[0033] One problem is that calculation of phase rotation amount due tothe static magnetic field non-uniformity in the 3PD method or 2PD methodwith the static magnetic field correction necessitates unwrappingprocessing, which requires a very long processing time.

[0034] The unwrapping processing will be explained hereinafter.

[0035] If phase has a value within a range from −π to +π, only one valuecan be defined. However, if the static magnetic field is nothomogeneous, the interval between TE1 and TE2 becomes large and at aposition where the phase value is −π or less, or +π or more, the phasevalue is wrapped to have a value between −π and +π. This situation isshown in FIG. 6, where abscissa is position and ordinate is phaserotation amount due to non-uniformity of the static magnetic field, and601 represents a distribution of non-uniformity of the static magneticfield in FOV.

[0036] As shown in FIG. 6, the portion where the phase value is +π ormore is wrapped to have a value 603. Similarly, portions where the phasevalues are −π or less is wrapped to have values 606, 607. This wrappingcauses discontinuity of phase value (illustrated by a dotted line in thefigure). Such discontinuity never occurs in an actual magnetic field andmust be eliminated to obtain a smooth static magnetic fieldnon-uniformity distribution by performing unwrapping processing. Amethod of unwrapping is described in the aforementioned paper and alsoin “Direct Calculation of Wrap-Free Phase Image”; M. Patel and X. Hu;Proceedings of Annual Meetings of the Society of Magnetic Resonance inMedicine (=SMRM), No. 721, 1993, and “Phase unwrapping in theThree-point Dixon Method for Fat Suppression MRI Imaging”; JerzySzumowski et al.; Radiology, Vol.192, 555-561(1994).

[0037] The unwrapping is a complicated and time consuming process sinceit is susceptible to noise and requires a technique of making masks foreliminating influence of noise. According to a preliminary examinationconducted by the inventors, processing time of about 20 s-30 s wasrequired to perform water/fat separation processing of 256×256 originalimages using a workstation.

[0038] In addition, unwrapping or other such correction of the staticmagnetic field is susceptible to noise, especially noise generated at aportion where the examined subject is not present or a border betweentissues in the subject. Such noise degrades the quality of whole imageand makes it difficult to obtain an accurate water/fat separated image.

[0039] Further, the 3PD method requires calculation of phase rotationamount due to non-uniformity of the static magnetic field and thiscalculation itself takes a long time.

[0040] Owing to this problem of long image processing time, it isdifficult to apply the water/fat separation technique to dynamic imagingfor time-course observation of one portion of the examined subject. Oneconceivable clinical application of dynamic imaging is to monitor aneedle inserted into a fat liver tumor using an MRI with considerablestatic magnetic field non-uniformity. In such a case, fat signals in theimage must be suppressed in order to depict the tumor with high contrastand yet the image must be updated at the speed of one or two images persecond. In dynamic imaging of coronary arteries, there is also a need todepict the surroundings of the coronary artery with high contrastcompared to fat. Further, in examination of the movement function oflimbs, which requires repeated scanning of the limbs while moving thejoint, there is also a need for monitoring water/fat-separated images insemi-real time. However, the aforementioned 3PD method and the 2PDmethod with the static magnetic field correction are not capable ofupdating images because of their long processing time.

[0041] Another problem in the 3PD method, especially in the 3PD methodusing a single scan sequence, is insufficient water/fat separation. Inexperiments by the inventors, it was found that water and fat were notsufficiently separated by performing the 3PD processing of data acquiredin a single scan sequence using an open-type MRI apparatus and that suchinsufficient separation is caused by a phase rotation component thatcannot be eliminated by the processing operations of the formulae(3)-(7).

[0042] Specifically, only the phase rotation component (α(x,y)), whichis the same for all of echoes, and the phase rotation component(α′(x,y), 2α′(x,y)), which are caused by non-uniformity of the staticmagnetic field and proportional to time, have been considered in theformulae. However, there is another type of a phase rotation component,which is not proportional to time and has different values for the threeechoes. Due to this component, water/fat separated images are notsuccessfully obtained. Such a phase rotation component having differentvalues for the three echoes includes one caused mainly by eddy currentsgenerated by inversion of a readout gradient magnetic field.

[0043] The present invention was accomplished in order to solve theabove-mentioned problems and an object of the present invention istherefore to provide an MRI apparatus and method for reconstructingplural images using plural images having different echo times or forobtaining a desired enhanced image by performing processing operationson a plurality of such signals, which is capable of producing water/fatseparated images having a high image precision by effectivelyeliminating influence of noise in image-processing operations. Anotherobject of the present invention is to provide an MRI apparatus andmethod capable of greatly reducing the time required for the water/fatseparation processing operations and thereby performing continuousimaging (dynamic imaging) effectively. Yet another object of the presentinvention is to provide an MRI apparatus and method capable ofeliminating influence of eddy currents which differs depending onsignals having different echo times to perform an accurate processingoperations between signals and thereby producing a plurality of imageshaving an excellent image quality.

DISCLOSURE OF THE INVENTION

[0044] An MRT apparatus according to the first embodiment of the presentinvention comprises means for selecting an enhanced image region to beformed by processing operations between signals for at least one imageformed using plural signals having different echo times. Since theprocessing operations between signals are performed on a limited regionto be observed, the amount of processing can be markedly reduced.

[0045] An MRI method of the present invention for obtainingwater/fat-separated images by acquiring original image data of pluralimages having different echo times and performing processing operationson the image data, comprises the steps of specifying a partial region ofthe original image data for the water/fat separation processing, andperforming the water/fat separation processing operations on thespecified region to produce water images or fat images of the region.

[0046] According to a preferred embodiment of the above MRI method,original image data for at least two images having different echo timesare acquired.

[0047] According to another preferred embodiment of the above-mentionedMRI method, a plurality of regions is specified as the partial regionfor water/fat separation processing.

[0048] According to another preferred embodiment of the above-mentionedMRI method, the original images are displayed on display means and thepartial region for water/fat separation processing is specified usingthe displayed original images. The water/fat-separated image isdisplayed on the specified partial region of the original image, and animage that is not water-far separated is displayed on the other region.

[0049] An MRI apparatus according to the first embodiment of the presentinvention comprises a signal detecting unit for detecting NMR signalsemitted from a subject to be examined, a signal processing unit forperforming image processing of the detected signals, a display unit fordisplaying the processed images and a control unit for controllingoperations of the signal detecting unit, signal processing unit anddisplay unit, and produces water/fat-separated images by acquiringoriginal data of plural images having different echo times andperforming processing operations on the original data, which apparatusfurther comprises means for specifying one or plural regions of theoriginal image data for performing the water/fat separation processing,wherein the signal processing unit performs the water/fat separationprocessing operations on the region specified by the specifying meansand the display unit displays water images or fat images produced by theprocessing operations.

[0050] According to a preferred embodiment of the above MRI apparatus, aregion of the original image obtained through a dynamic measurement isspecified and only the specified region is subjected to the water/fatseparation processing and displayed.

[0051] In the MRI apparatus of the first embodiment, the originalimages, which are not subjected to the water/fat separation processing,are displayed on the display unit and an operator determines a regionthat requires the water/fat separation processing. Thereafter, only datacontained in the specified region is subjected to the water/fatseparation processing. Such water/fat separation processing of thespecified region is applied to the dynamic measurement andwater/fat-separated images are displayed in real time.

[0052] According to the above-mentioned MRI apparatus, unnecessary dataof the original image is not used in the processing operations of thewater/fat separation processing. Thus, the amount of noise included inthe separation processing can be reduced to reduce errors in phaseunwrapping or other such water/fat separation processing. In addition,since the amount of data to be processed is reduced, the processing timecan be shortened.

[0053] An MRI apparatus according to a second aspect of the presentinvention controls readout gradient magnetic fields for generatingplural signals having different echo times so that all of signals aregenerated by applying the readout gradient magnetic fields having thesame polarity.

[0054] According to the second aspect, conditions of the phasefluctuation caused by eddy currents of the readout gradient magneticfield can be equalized for plural signals acquired in the same scan,whereby processing operations using the phase difference between thesignals can be performed accurately. This results in effective imageseparation utilizing the phase difference and production of plural kindsof images having an excellent image quality.

[0055] Specifically, the MRI apparatus according to the second aspect ofthe present invention, which acquires plural images having differentecho times by a single scan measurement and produces images byperforming processing operations, is characterized in that thepolarities of readout gradient magnetic fields for reading out the echosignals having different echo times are equalized.

[0056] According to a preferred embodiment of the MRI apparatus, arewind pulse having the polarity opposite to that of the readoutgradient magnetic field is applied prior to the readout gradientmagnetic field.

[0057] The present invention further provides an MRI apparatuscomprising means for generating a static magnetic field in a space wherea subject to be examined is placed, means for repeatedly applying an RFpulse for causing NMR in the subject, means for applying gradientmagnetic fields in the directions of slice, phase encode, and readout,receiver means for detecting plural echo signals emitted from thesubject, control means for controlling the aforementioned means so thatplural echo signals having different echo times are generated duringeach repetition time of the RF pulse, a readout gradient magnetic fieldis applied at the time when the echo signal is generated and phaseencode is changed at each repetition time, wherein the control meanscontrols the gradient magnetic fields so that a pulse having thereversed polarity of the readout gradient magnetic field is appliedahead of the readout gradient magnetic field.

[0058] In the above-mentioned MRI apparatus, the pulse may be appliedahead of the readout gradient magnetic field for the second andsubsequent echo signals in each repetition time. Alternatively, thepulse may be applied ahead of every readout gradient magnetic field forall of the echo signals.

BRIEF EXPLANATION OF DRAWINGS

[0059]FIG. 1 is a time chart of data acquisition in the 2PD method usinga GrE sequence.

[0060]FIG. 2 is a time chart of data acquisition in the 2PD method usingan SE sequence.

[0061]FIG. 3 is a time chart of data acquisition in the 3PD method usinga GrE sequence.

[0062]FIG. 4 shows a single-scan sequence of 3PD method using a GrEsequence.

[0063]FIG. 5 shows a single-scan sequence of 3PD method using an SEsequence.

[0064]FIG. 6 shows a static magnetic field non-uniformity distributionwithin a FOV.

[0065]FIG. 7 is an overall diagram of an MRI apparatus to which thepresent invention can be applied.

[0066]FIG. 8 is an explanatory view of region selection of an imagedisplayed on the display according to one embodiment of the presentinvention.

[0067]FIG. 9 illustrates a selected image displayed on the displayaccording to one embodiment of the present invention.

[0068]FIG. 10 shows an example of a fat-water separated image accordingto one embodiment of the present invention.

[0069]FIG. 11 shows another example of a fat-water separated imageaccording to one embodiment of the present invention.

[0070]FIG. 12 shows another example of a fat-water separated imageaccording to one embodiment of the present invention.

[0071]FIG. 13 shows a single-scan sequence (GrE sequence) of the 3PDmethod according to one embodiment of the present invention.

[0072]FIG. 14 shows a single-scan sequence (SE sequence) of the 3PDmethod according to one embodiment of the present invention.

[0073]FIG. 15 shows a single-scan sequence (GrE sequence) of the 3PDmethod according to another embodiment of the present invention.

[0074]FIG. 16 shows a single-scan sequence (SE sequence) of the 3PDmethod according to another embodiment of the present invention.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

[0075] Preferred embodiments of the present invention will be explainedhereinafter with reference to the attached drawings.

[0076] The configuration of an MRI apparatus to which the presentinvention is applied will be explained using FIG. 7. The MRI apparatuscomprises a magnet 702 which generates a static magnetic field in aspace surrounding a patient (subject to be examined) 701, a gradientmagnetic field coil 703 imparting gradient magnetic field to the space,an RF coil 704 which generates an RF magnetic field for producing NMR inatomic nuclear spins of atoms constituting tissues of the patient 701,an RF probe for detecting NMR signals emitted from the patient throughNMR.

[0077] The gradient magnetic field coil 703 consists of gradientmagnetic field coils of three directions X, Y, Z, each of whichgenerates a gradient magnetic field corresponding to signals from apower supply 709. The RF coil 704 generates an RF magnetic fieldcorresponding to signals from an RF transmitting unit 710. Signals fromthe RF probe 705 are detected by a signal detecting unit 706, processedby a signal processing unit 707 and transformed to image signals bycalculation. Images are displayed on a display 708. The gradientmagnetic field power supply 709, RF transmitting unit 710 and signaldetecting unit 706 are controlled by a control unit 711. A time chart ofthe control is generally called a pulse sequence. Various kinds of pulsesequences corresponding to imaging methods are preinstalled in thecontrol unit 711. A cradle 712 is provided for positioning the patientin the apparatus.

[0078] Although not illustrated in the figure, the control unit 711 isequipped with input means such as a keyboard, mouse and the like forinputting instructions to select a specific imaging method and/orvarious parameters. The selection of an imaging method and conditionsexplained hereinafter and the setting of processing operation conditionsare input through the input means.

[0079] In the MRI apparatus having such a structure, the subject 701 istransported into the homogeneous static magnetic field space formed bythe magnet 702, and thereafter an RF magnetic field having a frequencyproducing NMR in atomic nuclear spins (called simply spins hereinafter)of atoms constituting tissues of the patient is generated correspondingto signals from the RF transmitting unit 710. In this embodiment, thespins are those of the main constituent matter of the patient, i.e.,protons.

[0080] Next, an imaging method according to the first embodiment of thepresent invention will be explained with reference to FIG. 8-FIG. 12.The imaging method executes pulse sequences according to the 2PD methodor the 3PD method to obtain image data having different echo times. Thisprocess is the same as that of the conventional imaging method. However,one characteristic of this imaging method is that a one or more regionsare specified for water/fat separation processing when such processingoperations are performed on the image data and only water images or fatimages of the specified region or regions are produced.

[0081] One embodiment in which the imaging method of the presentinvention is applied to the 3PD method will be explained first. In thepulse sequence of the 3PD method, as shown in FIG. 3, a GrE sequence orSE sequence scan is conducted three times with the echo time TE set toTE1, TE2 and TE3. If the resonance frequency difference between thewater and fat due to the chemical shift is Δf and 2τ=1/Δf, the timedifference between TE1 and TE2 is set toe (2n+1)τ (where n is aninteger; the same applies hereinafter) and the time difference betweenTE1 and TE3 is set to 2nτ. Since signals generally attenuate with arelaxation time of T2, T2* or the like, TE2 and TE3 are set to TE1+τ andTE1+2τ respectively.

[0082] Instead of performing the scan three times, a multi-echo type SEor GrE sequence (single-scan sequence) shown in FIG. 4 or FIG. 5 may beemployed where the scan is performed once to generate three echoes. Useof such a single scan sequence reduces the measurement time to one thirdand, therefore, larger time reduction effect can be obtained.

[0083] In such a multi-echo type sequence, the polarity of the readoutgradient magnetic field is, for example, reversed alternately, asillustrated in FIG. 5 using symbols 506, 507 and 508, to obtain signals503, 504, 505 at TE of TE1, TE2, TE3, respectively. In this case, too,the interval of reversing the readout gradient magnetic field pulse isτ, and thus TE2, TE3 are set to TE1+τ, TE1+2τ, respectively.

[0084] In case of the three scans shown in FIG. 3, each scan is repeatedwhile changing a phase-encoding gradient magnetic field Gp. In case ofthe single-scan sequence shown in FIG. 4 or FIG. 5, the single-scansequence is repeated while changing the phase-encoding gradient magneticfield Gp. Thus, MR signals s1, s2, s3 of the required phase encodenumber for reconstructing of one image can be obtained.

[0085] Next, the processing operations performed on the thus obtained MRsignals will be explained.

[0086] First, Fourier transform is performed on the MR signals s1, s2and s3 obtained by measurements of TE1, TE2 and TE3 to produce imagedata S1, S2 and S3. The obtained image data S1, S2 and S3 are complexdata. In this embodiment, absolute value images (magnitude images) ofthe image data S1, S2 and S3 are displayed on the display 708. Thedisplayed images may be any one of image data S1, S2 and S3 or anycombination of them. FIG. 8 illustrates the case where only an image 800of image data S1 is displayed.

[0087] When the image 800 is displayed on the display 708, a region onwhich the water/fat separation processing is to be performed (subjectregion) is designated on the displayed image 800. This designation canbe made, for example, by drawing a border enclosing the subject region801, which is a region constituting one part of the image 800, using andelineating tool such as a mouse pointer or a pen. Information on thethus indicated subject region (pixel address) is input into the signalprocessing unit 707. The number of subject regions is not limited to onebut may be two or more. In such a case, information on the pluralsubject regions is read in the order drawn, or according the priority ofthe subject regions indicated separately.

[0088] When two images (A) and (B) for image data S1 and S2 aredisplayed on the display 708 as shown in FIG. 9, the subject region isindicated on either of (A) and (B) in the same manner as explainedabove. In this case, two subject regions 801 and 802 (or 803 and 804) orplural subject regions may be indicated for one image. When pluralsubject regions are selected, they are read in the order drawn oraccording to the separately indicated priority.

[0089] While FIG. 8 and FIG. 9 illustrate that selection of the subjectregion is conducted by freehand drawing, a square subject region may bedesignated by selecting diagonally opposite corner points of the square,or a circular region can be selected. Techniques known to the art can beemployed to a region in a displayed image.

[0090] Once the subject region is thus indicated, the signal processingunit 707 performs the water/fat separation processing for pixelscorresponding to the indicated region. Specifically, a static magneticfield non-uniformity distribution map is formed by the following formula(6) using image data S1 and S3.

arg(S3(x,y)/S1(x,y))=2α′(x,y)  (6)

[0091] Next, the unwrapping processing is performed for eliminatingphase wrapping and then a phase rotation amount α′(x,y) due to staticmagnetic field non-uniformity is found by dividing the processed data by2. Image data S1 and S2 are phase-corrected by the following formulae(7) and (8) using the phase rotation amount α′. This phase correction isconducted only for the previously designated subject region for thewater/fat separation processing.

S1′(x,y)=S1(x,y)exp(−i2α═(x,y))  (7)

S2′(x,y)=S2(x,y)exp(−i(2n+1)α′(x,y))  (8)

[0092] By this correction, signals S1′ and S2′ whose phase rotations dueto the static magnetic field non-uniformity are corrected can beobtained and an addition operation or subtraction operation of the aboveformulae (7) and (8) gives water image W(x,y) as an addition image orfat image F(x,y) as a subtraction image.

S1′(x,y)+S2′(x,y)=2W(x,y)

S1′(x,y)−S2′(x,y)=2F(x,y)

[0093] If the SE sequence is employed, since only the phase rotationstarting from acquisition of S1 need be corrected, addition orsubtraction of the image data S1 and the corrected S2′ is performed toproduce water/fat separated images.

[0094] The thus obtained water image and fat image are displayed on thedisplay 708. FIG. 10 illustrates an image obtained by performing thewater/fat separation processing on the subject region indicated on theimage 800 shown in FIG. 8. Here, (A) is a water-image and (B) is afat-image. Either or both of the water image and fat image may bedisplayed. As for a portion other than the subject region, it ispreferable for the non-processed image to be displayed at a loweredcontrast because this clarifies how the water/fat separation processedportion is related to the whole image.

[0095]FIG. 11 and FIG. 12 illustrate examples of displayed images. Inthe examples shown in FIG. 11 and FIG. 12, a water/fat separationprocessed image, water image or fat image, is displayed for theindicated region 801 and a non-processed image is displayed for theregion 802 other than the designated region. Specifically, in theexample shown in FIG. 11, a water image is displayed in a region 801 ofthe original image 800, and image of the other portion 802 displays theimage in the original state. In the example shown in FIG. 12, a fatimage is displayed in a region 801 of the original image 800 and imageof the other portion 802 displays the image in the original state.

[0096] By displaying the separation-processed image together with theoriginal image, the position of the water/fat separation in the wholeimage can be recognized by a glance, and comparison of the water/fatprocessed region and the other portion is facilitated.

[0097] The imaging method according to the first embodiment of thepresent invention has been explained for the application to the 3PDmethod. The imaging method can be applied to the 2PD method similarly.The application of the present invention to the 2PD method accompaniedby a static magnetic field correction now will be explained.

[0098] In this case, the measurement is performed using an SE sequenceor GrE sequence similarly to the 3PD method. As shown in FIG. 1 and FIG.2, the scan is conducted twice with the echo time TE set to differenttimes TE1 and TE2. Alternatively, a multi-echo type SE or GrE sequenceis conducted to generate two echoes within each repetition and the scanis conducted once. The measurement acquiring two echoes during a singlescan reduces the measurement time, which is very beneficial for users.

[0099] In any case, the time difference between TE1 and TE2 is set to(2n+1)τ and, ordinarily TE2 is set to TE1+τ.

[0100] The MR signals s1 and s2 thus obtained at different echo timesTE1, TE2 are subjected to Fourier transform to produce image data S1 andS2. Since the image data S1 and S2 are complex data, absolute value(magnitude) images are displayed on the display 708. The imagesdisplayed here may be either or both of S1 and S2. Next, a userindicates a region to be water/fat separation-processed on the displayedimage by drawing a border enclosing the region using a delineating toolsuch as a mouse pointer or pen.

[0101] Once the subject region has be designated, water/fat separatedimages are found only for the subject region according to theaforementioned 2PD method with a static magnetic field correction in themanner same as in the application to the 3PD method, and displayed onthe display 708. Regarding the manner of displaying images, the subjectregion may be displayed alone or, preferably, together with the originalimage for the region other than the subject region as explained withrespect to the 3PD method.

[0102] As previously mentioned, the 2PD method with a static magneticfield correction requires more complicated processing and a longerprocessing time than the 3PD method since it uses signals in which waterspins and fat spins are out-phase. Accordingly, the measurement timereduction effect of the present invention is especially advantageous.The measurement time reduction effect brought about by the inventionamounts to, for example, a reduction to about ¼ when the length of eachside of a square subject region is ½ that of the original image.

[0103] The aforementioned imaging method according to the firstembodiment can be applied to not only the measurement of a single imagebut also plural images obtained by dynamic imaging or multi-sliceimaging.

[0104] Dynamic imaging is an imaging method where the same portion ofthe patient is measured at regular intervals to obtain a series ofimages showing change in the portion over the course of time. In case ofdynamic imaging, a magnitude image obtained by the first scan isdisplayed, and a region to be water/fat separation processed isdetermined on the magnitude image. In the second and later scans, animage which is partially water/fat separation processed for thedetermined region is produced and displayed in real time.

[0105] In the conventional technique, since it takes a long time, 20s-30 s, to perform the water/fat separation process, it is difficult toproduce water/fat separated images corresponding to the acquired imagesone by one (i.e., real time images). However, the water/fat separationprocessing can be performed within several seconds by applying theimaging method of the present invention to the dynamic imaging, andwater/fat separated images corresponding to the acquired images one byone can be produced.

[0106] While either of the 3PD method or 2PD method mentioned earliermay be employed for the dynamic imaging, the best real timecharacteristic can be obtained when a two-echo GrE sequence (2PD) havinga short measurement time is employed.

[0107] In addition, the location of the inspection site may changeduring a time series of data is collected in the dynamic imaging. Tocope with such a case, a design that enables the subject region to bereset and set through GUI as an occasion demands is preferable. By this,any movement of the inspection site can be easily dealt with.

[0108] A multi-slice imaging is a method which acquires MR signals fromplural slices during one repetition time TR. In this case, an absoluteimage of one slice is displayed first and the subject region isdetermined on the displayed image. Then the determined region is usedfor all of the slices to perform the water/fat separation processing.Alternatively, the subject region may be determined for each slice.Since the water/fat separation processing is performed after themeasurement, determination of the subject region can be done withouttime constraint and designed to be switched arbitrarily by the user.

[0109] As mentioned earlier, the first embodiment of the presentinvention is configured such that, in an MRI apparatus and method forobtaining water/fat separated images by acquiring a plurality oforiginal image data having different echo times and performingoperations on the original data, a partial region of the original imagedata is specified and the water/fat separation processing is performedfor the specified region.

[0110] Since noise components included in the specified region are fewerthan those included in the original image data, noise-induced errors inthe processing operations, e.g., phase unwrap processing, can be reducedto suppress image degradation and enable production of improved-qualitywater/fat separated images.

[0111] In addition, since the data volume for the water/fat separationprocessing is reduced, the time required for the water/fat separationprocessing can be reduced. Accordingly, there can be provided an MIapparatus and method capable of producing water/fat separated imageshaving improved image accuracy in a short processing time.

[0112] Next, a second embodiment of the present invention will beexplained with reference to FIG. 13-FIG. 16.

[0113] This embodiment is characterized in that, when images such aswater/fat separated images are produced by performing processingoperations between plural images having different echo times, polaritiesof read-out gradient magnetic fields for generating echo signals havingdifferent echo times are equalized so that components not proportionalto time are not appended to the individual echo signals, therebyimproving image quality.

[0114] This embodiment is also applied to the MRI apparatus having anoverall configuration as shown in FIG. 7. Here, however, a single-scansequence of acquiring multiple echoes in a single scan is conducted, andthe control unit 711 effects control such that the polarities ofgradient magnetic fields for generating echo signals are equalized forechoes acquired in the scan.

[0115] An application of the present invention employing a GrE typesingle scan sequence now will be explained hereinafter. According tothis sequence, as shown in FIG. 13, an RF exciting pulse (RF) and agradient magnetic field (Gs) are applied, and then a readout gradientmagnetic field pulse (pre-pulse) 1306 is applied in the negativedirection. Thereafter a readout gradient magnetic field pulse 1301 isapplied in the positive direction to generate a first echo signal 2τafter application of the RF exciting pulse. Next, a second echo signalis generated τ after generation of the first echo signal by applying areadout gradient magnetic field pulse rewind pulse) 1304 in the negativedirection and a readout gradient magnetic field pulse 1302. Finally, agradient magnetic field pulse (rewind pulse) 1305 in the negativedirection and a readout gradient magnetic field 1303 are applied togenerate a third echo signal τ after the generation of the second echosignal. The application of the RF exciting pulse and gradient magneticfields is repeated plural times (for example 256 times) while changingthe phase encode to produce three kinds of image data having differentecho times. By thus applying the rewind pulses 1302, 1305, thepolarities of the readout gradient magnetic fields 1301, 1302, 1303 forgeneration of echo signals can be equalized.

[0116] The application times of the readout gradient magnetic fields1301, 1302, 1303 can be longer in proportion as the intensities of thegradient magnetic fields for these rewind pulses are larger and theapplication times are shorter. Therefore, the receiving bandwidth can benarrowed and the SN ratio can improved. Accordingly, it is preferred tomake the pulse intensities of the pre-pulse 1306 and rewind pulses 1304,1305 high and to make the pulse widths narrow.

[0117] The rewind pulses 1304, 1305 have polarities opposite to thereadout gradient magnetic fields 1302, 1303 and the same area as thereadout gradient magnetic fields 1302, 1303. The pre-pulse 1306 has thesame polarity and the same intensity as the readout gradient magneticfield 1304 but a half pulse width. So far as these relationships aremaintained, the polarities shown in the figure may be reversed, i.e.,the pre-pulse 1306 and rewind pulses 1304, 1305 may be applied in thepositive direction and the readout gradient magnetic fields 1301, 1302,1303 may be applied in the negative direction.

[0118] Next, another application of the present invention employing anSE type single scan sequence will be explained with reference to FIG.14. In this case, an RF exciting pulse and a slice gradient magneticfield are applied and then an inversion RF pulse is applied at ½ echotime. Then, after a gradient magnetic field (pre-pulse) 1406 is applied,a readout gradient magnetic field 1401 is applied in the positivedirection to generate a first echo signal TE after application of the RFexciting pulse. Next, gradient magnetic field rewind pulse) 1404 in thenegative direction and a readout gradient magnetic field 1402 areapplied again to generate a second echo signal τ after generation of thefirst echo signal. Finally, a readout gradient magnetic field (rewindpulse) 1405 in the negative direction and a readout gradient magneticfield 1403 are applied to generate a third echo signal τ aftergeneration of the second echo signal. The application of the RF excitingpulse and gradient magnetic fields is repeated plural times (for example256 times) while changing phase encode to produce three kinds of imagedata having different echo times.

[0119] By applying the rewind pulses 1404 and 1405, the polarities ofthe readout gradient magnetic fields 1401, 1402, 1403 for generation ofecho signals can be equalized, similarly to the application to the GrEtype single-scan sequence.

[0120] Similarly to the previous embodiment, so far as the rewind pulses1404 and 1405 have polarities opposite to and areas equal to the readoutgradient magnetic fields 1302 and 1303, and the pre-pulse 1306 has apolarity and intensity equal to the readout gradient magnetic field 1304but a half pulse width, the pre-pulse 1406 and rewind pulses 1404, 1405may be applied in the positive direction and the readout gradientmagnetic fields 1401, 1402, 1403 may be applied in the negativedirection.

[0121] By equalizing the polarities of the readout gradient magneticfields 1301-1303 and 1401-1403, influence of eddy currents caused byinversion of a gradient magnetic field can be suppressed. This enhancesthe accuracy of the water/fat separation processing to produce excellentwater/fat separated images.

[0122] In FIG. 13 and FIG. 14, water signals and fat signals in thefirst echo are in phase (same phase), out of phase (different phase) inthe second echo and in phase in the third echo. Since the second echo isout of phase, water signals and fat signals cancel each other so thatsignal intensity is lowered in a portion where water and fat co-exist.On the other hand, since the first and third echoes are in phase, thesignals can have high intensity without canceling each other even in aportion where water and fat co-exist. Accordingly, when the staticmagnetic field non-uniformity map is formed using the first echo andthird echo, an accurate static magnetic field non-uniformity map can beproduced even in a portion where water and fat co-exist. This is veryimportant for obtaining water/fat separated image stably. According toan investigation conducted by the inventors, stable water/fat imagescould be obtained even if the first echo was slightly out of phase(about π/10 shifted from the in-phase state) and such a shift wastolerable.

[0123] The first to third echoes may be acquired as out-of phase,in-phase and out-of phase echoes by properly determining the acquisitiontime of the first echo from the RF exciting pulse or RF inversion pulse.In this case, too, influence of eddy currents caused by inversion of agradient magnetic field can be suppressed. However, an experimentconducted by the inventors showed that acquisition of echoes in theorder of in-phase, out-of-phase and in-phase resulted in more stableprocessing and was effective.

[0124] The value of τ in FIG. 13 and FIG. 14 is expressed by 1/2γσB(τ=1/2γσB) using the relations of Δf=γσB (σ is the chemical shiftdifference between water and fat) and 2τ=1/Δf, which decreases ininverse proportion to the static magnetic field intensity. Accordingly,this embodiment is more suitable for the middle or low magnetic fieldclass MRI apparatus than for the high magnetic field class MRIapparatus. Typically, the embodiment is easily realized in the staticmagnetic field intensity of 0.2T-0.5T, especially about 0.3T. If thetechnique of the present invention is applied to an MRI apparatus of0.3T, τ will be 11.4m. If the magnetic field intensity becomes high andτ becomes short, the rise time and fall time of the gradient magneticfield pulse increase relatively and the time of signal acquisition isshortened. In order to collect required data during the measurementtime, it is necessary to shorten the sampling interval. As a result, thereceiving bandwidth becomes wide and the S/N becomes low. In order toshorten the rise time and fall time of the gradient magnetic field, therising characteristic of the gradient magnetic field should be improved.

[0125] The inventors conducted an experiment on water/fat separationaccording to this embodiment of the present invention using a 0.3Topen-type MRI apparatus. The apparatus was equipped with a permanentmagnet and pole pieces for generating the static magnetic field, and hada vertical magnetic field system and an asymmetrical two-post structure.The slew rate of the gradient magnetic field was 20T/m/s. In the imagingsequence of the experimental imaging, both of the rise time and falltime of the readout gradient magnetic field was 850 μs. The signaldetecting band was 35 kHz (FOV=350 mm) or 45 kHz (FOV=200m) and thematrix size was 256×256. Under such imaging conditions, imagingaccording to the embodiment was realized easily.

[0126] Use of rewind pulses in an echo planar imaging (EPI) using a highmagnetic field MRI is described in “Echo-Planar Imaging with AsymmetricGradient Modulation and Inner-Volume Excitation”; D. A. Feinberg et al.;Magnetic Resonance in Medicine, Vol. 13, 162-169 (1990). EPI is alsosusceptible to the phase rotation caused by inversion of a readoutgradient magnetic field. However, in EPI, plural echo signals havingdifferent echo times are differently phase-encoded in order to produceone image quickly. The imaging of the present invention is differentfrom the EPI in that echo signals having different echo times areacquired to collect echo signals having the same echo time of a numberequal to a repetition number and to produce three images. It is alsodifferent in that echo signals having different echo times generated inthe same TR are imparted with the same phase-encode. Yet, effect of therewind pulse is different. Specifically, in the embodiment of thepresent invention, the rewind pulse is used for matching between threeimages having different characteristics, whereas the rewind pulse isused for matching between signals of one image in EPI. Accordingly, theembodiment is not a simple application of the rewind pulse of EPI.

[0127] A technique of producing water/fat separated images fromin-phase, out-of-phase and in-phase data in a single scan sequence isdisclosed in “Separation of water and fat MR images in a single scan at0.35 T using “Sandwich” echoes”, W. Zhang et al., Journal of MagneticResonance Imaging, Vol. 6, no. 6, PP 909-917, 1996. Although thisarticle discloses inversion of the polarities of the first and thirdechoes, it does not disclose that polarities of all of echo signals areequalized as in the present embodiment.

[0128] Another embodiment of the present invention now will beexplained. In this embodiment, the sequences shown in FIG. 13 and FIG.14 are modified so that influence of readout gradient magnetic fieldsapplied at acquisition of echo signals is equalized for the first echo,second echo and third echo. In the following figure, the same symbols asin FIG. 13 and FIG. 14 are used for the same components.

[0129] One application employing a GrE type single scan sequence isillustrated in FIG. 15. In the sequence shown in FIG. 15, a rewind pulse1506 having an application time and intensity equal to the rewind pulses1304, 1305 is applied instead of the pre-pulse 1306 of FIG. 13, and apre-pulse having a polarity opposite to the rewind pulse 1506 and a halfpulse area is added ahead of the rewind pulse 1506. By applying thepre-pulse 1507, a half area of the rewind pulse 1506 is offset and, as aresult, readout gradient magnetic fields 1301, 1302 and 1303 can beapplied in the same polarity similarly to the sequence shown in FIG. 13.In addition, since the intensity and application time of the rewindpulses 1304, 1305 and 1506 are equalized by applying the pre-pulse 1507,influence of readout gradient magnetic fields applied at acquisition ofechoes, i.e., influence of eddy currents caused by inversion of thepolarity of the gradient magnetic field, is equalized for the firstecho, second echo and third echo.

[0130] Another application using an SE type single scan sequence isshown in FIG. 16. In the sequence of FIG. 16, a rewind pulse 1606 havinga longer application time is applied instead of the pre-pulse 1406 ofFIG. 14, and a pulse 1607 having the same polarity as the rewind pulse1606 and a half area is added ahead of the rewind pulse 1606. This makesit possible not only to apply readout gradient magnetic fields 1401,1402, 1403 with the same polarity but also to equalize the intensity andapplication time of the rewind pulses 1606, 1404 and 1405. Similarly tothe aforementioned GrE type, influence of readout gradient magneticfields applied at acquisition of echoes, i.e., influence of eddycurrents caused by inversion of the polarity of gradient magneticfields, is equalized for the first echo, second echo and third echo.

[0131] Specifically, in the embodiments of FIG. 13 and FIG. 14, althoughthe application time and intensity of readout gradient magnetic fields1301-1303, 1401-1403 are the same for the first echo, second echo andthird echo, pulses applied ahead of them (pre-pulses 1306, 1406 andrewind pulses 1304, 1305, 1404, 1405) are not identical and imagedegradation might occur owing to disturbance of phase rotationcomponents due to application of the thus different gradient magneticfields. However, if the pulses applied ahead of the readout gradientmagnetic fields 1301-1303, 1401-1403 are equalized as aforementioned,such problem can be solved.

[0132] Since the polarities of all of the readout gradient magneticfield 1301-1303, 1401-1403 are equalized and the application times andintensities of the readout gradient magnetic field 1301-1303, 1401-1403which are applied ahead of the rewind pulses 1506, 1304, 1305, 1606,1404, 1405 are equalized, disturbance owing to the influence ofapplication of gradient magnetic fields for the first-third echoes canbe suppressed. Since influence of eddy currents caused by inversion ofgradient magnetic fields and disturbance thereof can be thus suppressed,accurate water/fat separation processing can be done to produceexcellent water/fat separated images.

[0133] In FIG. 15, the height of the pre-pulse 1507 may be lower thanthat of the rewind pulse 1506. Similarly, in FIG. 16, the height of thepre-pulse 1607 may be lower than that of the rewind pulse 1606. If theheight of a pulse is lowered, the pulse area is maintained thepredetermined value by prolonging the application time.

[0134] Further, application of the pre-pulse 1607 ahead of the second RFpulse in FIG. 16 is effective for shortening TE. The reason is that,although the pre-pulse 1607 can be applied after the second RF pulse asa sequence design, reduction of TE is restrained in this case becausetwo pulses including the pre-pulse 1607 and the rewind pulse 1606 areapplied during the last half of TE (TE/2). In addition, it becomesnecessary to apply the pre-pulse with a reversed polarity.

[0135] In FIG. 13-FIG. 16, it is preferable to make the height of therewind pulses 1304, 1305, 1404, 1405, 1506 and 1606 as high as possible.For example, in an MRI apparatus implemented with a gradient magneticfield system of the maximum gradient magnetic field of 15 mT/m, it maybe more than about 14 mT/m. As a result, the application time of thereadout gradient magnetic fields 1301-1303 and 1401-1403 can be long,the receiving bandwidth can be narrow and the image S/N can be improved.

[0136] The embodiments shown in FIG. 13-FIG. 16 are effective especiallyin an MRI apparatus influenced considerably by eddy currents generatedby application of gradient magnetic fields, e.g., an MRI apparatusequipped with pole pieces having a large residual magnetic field andeddy currents.

[0137] The second embodiment applied to the Dixon method has beenexplained in the foregoing. This embodiment can be applied to imagingmethods other than the Dixon method. Typical of these imaging methods isthe method of Qing-San Xiang et al., in which plural image data havingdifferent TE are acquired, and water signals and fat signals areseparated by performing processing operations on the acquired image datato produce images. Further, this embodiment can be applied to not onlyseparation of water signals and fat signals but also to processionoperations on plural image data having different TE to improve precisionof the operations.

[0138] According to the second embodiment, when plural image data havingdifferent echo times are acquired, components not proportional to timecan be eliminated from each echo. This makes it possible to accuratelyimplement a method of water/fat separation imaging by processingoperations and to produce excellent water/fat-separated images.

1. A magnetic resonance imaging method of producing water/fat-separatedimages by acquiring original image data of plural images havingdifferent echo times and performing processing operations thereon, whichmethod includes the steps of specifying a partial region of the originalimage data to be subjected to water/fat separation processing operationsand performing the water/fat separation processing operations on thespecified region to produce water images or fat images thereof.
 2. Themagnetic resonance imaging method of claim 1, wherein the acquiredoriginal image data having different echo times consist of image data ofat least two images.
 3. The magnetic resonance imaging method of claim1, wherein a plurality of regions can be specified for the water/fatseparation processing.
 4. The magnetic resonance imaging method of claim1, which method further includes the steps of displaying the originalimage on a display means, specifying the partial region for perform thewater/fat separation processing through the displayed original image,displaying a water/fat-separated image for the specified region and anon-processed image for a region other than the specified region.
 5. Themagnetic resonance imaging method of any one of claims 1-3, whereinacquisition of the original image data is conducted continuously and thewater/fat-separated image is displayed and updated in real time.
 6. Themagnetic resonance imaging method of claim 5, wherein the step ofspecifying of the partial region for the water/fat separation processingis conducted at arbitrary time during continuous acquisition of theoriginal image data.
 7. A magnetic resonance imaging apparatuscomprising a signal detecting unit for detecting NMR signals emittedfrom a subject to be examined, a signal processing unit for performingimage processing on the detected signals, a display unit for displayingthe signal-processed images, a control unit for controlling operationsof the signal detecting unit, the signal processing unit and the displayunit, and producing water/fat-separated images by acquiring pluraloriginal image data of plural images having different echo times andperforming processing operations thereon, wherein a partial region ofthe original image data for performing the water/fat separationoperation is specified through the display unit and is subjected to thewater/fat separation processing operations by the signal processingunit, and the water/fat separation processed water images or fat imagesare displayed on the display unit.
 8. A magnetic resonance imagingapparatus comprising a signal detecting unit for detecting NMR signalsemitted from a subject to be examined, a signal processing unit forperforming image processing on the detected signals, a display unit fordisplaying the signal-processed images, a control unit for controllingoperations of the signal detecting unit, the signal processing unit andthe display unit, and producing water/fat-separated images by acquiringoriginal image data of plural images having different echo times andperforming processing operations thereon, which apparatus furthercomprises means for specifying one or plural regions for performing thewater/fat separation processing, wherein the signal processing unitperforms the water/fat separation processing on the region selected bythe specifying means and the display unit displays the processed waterimages or fat images.
 9. The magnetic resonance imaging apparatus ofclaim 8, wherein the display means displays the water images or fatimages superimposed on the original images.
 10. A magnetic resonanceimaging apparatus for producing images by acquiring plural images havingdifferent echo times by a single scan measurement and performingprocessing operations thereon, wherein the polarities of readoutgradient magnetic fields for acquiring echo signals are equalized forecho signals having different echo times.
 11. The magnetic resonanceimaging apparatus of claim 10, wherein a rewind pulse having a polarityopposite to that of the readout gradient magnetic field is applied aheadof the readout gradient magnetic field.
 12. A magnetic resonance imagingapparatus comprising means for generating a static magnetic field in aspace where a subject to be examined is placed, means for repeatedlyapplying an RF pulse to produce NMR in the subject, means for applyinggradient magnetic fields in the slice direction, in the phase encodedirection and in the readout direction, receiving means for detectingplural echo signals emitted from the subject, control means forcontrolling the said means such that plural echo signals are generatedat different echo times during each RF pulse repetition time by applyingreadout gradient magnetic fields while changing phase encode at eachrepetition, and processing means for producing images by performingprocessing operations on the echo signals having different echo times,wherein the control means effects control such that a pulse with apolarity opposite to the readout gradient magnetic field is appliedahead of the readout gradient magnetic field.
 13. The magnetic resonanceimaging apparatus of claim 12, wherein the control means effects controlsuch that the pulse is applied ahead of readout gradient magnetic fieldsfor the second and subsequent echo signals in each repetition time. 14.The magnetic resonance imaging apparatus of claim 12, wherein thecontrol means effects control such that the pulse is applied ahead ofreadout gradient magnetic fields for every echo signals.